For a nervous system to be properly wired, the axons have to be guided toward the correct targets and the dendrites need to have the correct branching pattern and structural specialization. At present, much less is known about the molecular mechanisms that control dendrite development as compared to those controlling axon guidance. A few years ago, our lab initiated a very fruitful genetic dissection of dendrite development using Drosophila Multiple Dendritic neurons as a model system. This ongoing genetic screen has begun to yield important insights about the molecular basis of dendrite development in Drosophila. Given the striking conservation of many molecular mechanisms that control various developmental processes including axon guidance, it is highly likely that many of the molecular mechanisms controlling dendrite development are conserved between Drosophila and mammals. Indeed, we already have considerable success in our ongoing efforts to extend our findings from Drosophila to mammalian CNS. We propose to focus on the group of dar (dendritic arborization reduction) genes. Mutations of any of the dar genes lead to defective dendrtic arbor but normal axonal projections. Thus, studies of dar genes should reveal how axons and dendrites are made differently, which is of fundamental importance in understanding dendrite development. From our mutant screen, we estimate that there may be a total of about 20 dar genes in Drosophila. Of the 5 dar genes that we have cloned so far, all five have mammalian homologues. Remarkably, 3 encode components of the secretory pathway. These results reveal the preferential role of ER-Golgi trafficking and Golgi outposts in dendrite arborization. In two test cases, both mammalian homologues have dendrite specific developmental function. Thus, we believe the Dar genes will be instrumental in revealing evolutionarily conserved mechanisms in regulating dendrite but not axon morphogenesis. We propose to systematically identify mammalian homologues of Drosophila dar genes. We will use cell biological and genetic techniques to study the function of mammalian Dar genes in cultured rat hippocampal neurons with the emphasis on the role of ER/Golgi trafficking and Golgi outposts in dendrite development. Additionally, we will study the in vivo roles of the most interesting Dar genes in dendrte development of mammalian CNS by using our newly developed method to knockout genes in a spatially and temporally controlled manner. This work will contribute to the understanding and eventual treatment of human neurological diseases many of which have pathology in dendrites. PUBLIC HEALTH RELEVANCE: This project aims to unravel the molecular mechanisms that control dendrite development in mammalian central nervous system. Given that dendrite defects are the likely cause of many mental disorders such as autism, this project could contribute to the diagnosis and possible treatment in the future.